An antenna reflector array (or ‘reflectarray antenna’) comprises a set of radiating phase-shifting cells assembled in a one- or two-dimensional array and forming a reflecting surface allowing the directivity and gain of the antenna to be increased. The radiating phase-shifting cells of the reflector array, of the metal patch type and/or slot type, are defined by parameters able to vary from one cell to another, these parameters being for example the geometrical dimensions of the etched patterns (length and width of the patches or the slots) which are adjusted in such a manner as to obtain a desired radiation diagram.
The radiating phase-shifting cells can be formed by metal patches loaded with radiating slots and separated from a metal ground plane by a distance typically in the range between λg/10 and λg/6, where λg is the guided wavelength in the spacer medium. This spacer medium can be a dielectric material, but also a composite multilayer formed by a symmetrical arrangement of a separator of the honeycomb type and of thin-film dielectric layers. For an antenna to have a high performance, the elementary cell must be able to precisely control the phase-shift that it produces on an incident wave, for the various frequencies within the bandwidth. It is also a requirement that the process of fabrication of the reflector array be as simple as possible.
For this purpose, the applicant has previously filed a first French Patent application FR 0450575 entitled “Phase-shifting cell with linear polarization and with a variable resonant length using MEMS switches”. FIG. 1 shows an embodiment of this type of phase-shifting cell CD. Its principle of operation consists in modifying the electrical length of the slot FP by placing one or more variable and controlled localized loads DC′ in several different states allowing and disallowing the establishment of a short-circuit. The variation of the characteristic resonant length of the cell allows a modification of the phase-shift of the waves to be reflected. For an antenna, the waves originate from the RF source. A cell according to FIG. 1 comprises a substrate SB having a back face rigidly attached to a ground plane.
This phase-shifting cell only works for one linear polarization of the incident wave. Furthermore, the size of the cell is relatively large, of the order of 0.7λ, where λ denotes the wavelength. The mesh size of the reflector array, in other words the spatial periodicity according to which the cells are arranged in an array, is therefore much greater than 0.5 λ. This results in a non-optimal behaviour for very oblique incidences of the wave, associated with the possibility of excitation of a higher-order Floquet mode. This effect leads to a degradation of the side-lobes of the radiation diagram, also denoted by those skilled in the art as the “lobe image”.
The phase-shifting cell mainly functions as a patch-type resonance modulated by the electrical length of the slot or slots. The attainment of a phase cycle greater than 360° by the modulation of this single resonance is a critical point, and certain phase states are achieved by highly resonant configurations of the phase-shifting cell. These highly resonant configurations are also characterized by higher losses, together with a higher sensitivity of the electrical characteristics to the fabrication tolerances of the cell and of the variable and controlled localized loads.
The applicant has filed a second French patent application entitled “Reflector array with optimized arrangement and antenna comprising such a reflector array”. It has a phase cycle produced by phase-shifting cells having an internal structure that has a progressive development from one phase-shifting cell to another adjacent phase-shifting cell, and thus not introducing significant disruptions in periodicity over the reflecting surface. This type of cell thus avoids the interference induced in the radiation diagram by a spurious diffraction phenomenon on regions with abrupt disruptions in periodicity. FIG. 1b shows one example of a periodic pattern comprising a one-dimensional arrangement of several elementary radiating elements that allows a phase rotation of 360° to be obtained. It has the property of having the identical end phase-shifting cells of the phase cycle. A progressive phase cycle has also been included using a phase-shifting cell with variable and controlled localized loads.
FIG. 2 shows the layout of a radiating phase-shifting cell for such a reflector array. According to one embodiment, this phase-shifting cell takes the form of a cross with two perpendicular branches. The cross comprises three concentric annular slots 81, 82 and 83 formed in a metal patch. Variable and controlled localized loads 85 are disposed in a chosen fashion within the slots and allow the electrical length of the slots, and hence the phase of a wave reflected by the phase-shifting cell, to be varied. With several cells, it is possible to form a pattern with progressive phase variation and not comprising any abrupt transition on the surface of a reflector, by using several radiating elements having the same geometry, the same number of MEMS positioned at the same place in the annular slots, but MEMS being configured in different states. For example, with a pattern composed of several radiating elements in the form of a cross or a hexagon, having three concentric annular slots and with a MEMS in each slot, it is possible to make the phase vary progressively up to 1000° by progressively short-circuiting the various slots of the adjacent radiating elements until a radiating element having all its MEMS in the closed state is obtained, then over several additional adjacent elements, in progressively setting the MEMS in the open state until a radiating element having all its MEMS in the open state is obtained.
Although it is possible to produce a phase cycle greater than 360°, and having the same initial and final phase-shifting cell of the cycle, it is very difficult to obtain these phase states with cells having little resonance. A large number of resonant modes can potentially be excited, owing to the presence of several resonators. The appearance of these resonant modes can lead to an abrupt variation in the phase as a function of frequency. The rapid variations in the phase result in significant losses, in particular when ohmic MEMS are used, and in a sensitivity to the dispersions in fabrication of the MEMS.